The present invention relates to processing seismic reflection data and more particularly, relates to the adjustment of seismic reflection data to be consistent with other data, such as seismic data, well-logs, or VSP traces.
Conventional land or marine seismic acquisition techniques involve the use of an appropriate source to generate seismic energy and a set of receivers, spread out along or near the surface of the earth on land or at or near the water surface or water bottom in a water covered area, to detect any reflected seismic signals due to seismic energy striking subsurface geologic boundaries. These signals are recorded as a function of time and subsequent processing of these time varying signals, i.e., seismic "traces" or seismic data, is designed to reconstruct an appropriate image of the geologic boundaries of the subsurface and to obtain information about the subsurface materials. In general terms, this conventional process has a seismic wave, from a source of seismic energy, traveling down into the earth, reflecting from a particular geologic interface (i.e., a change or contrast in elastic constants and/or densities), and returning to the surface, where it may be detected by appropriate receivers. In this specification reference is made to a subsurface feature; this means a particular seismic reflection or group of reflections representative of a specific geologic interface or grouping of geologic interfaces on a seismic trace.
It is generally the objective of seismic exploration to generate seismic energy, make measurements of the reflection amplitude of this energy at various offsets and for various times, and then, by employing various processing steps on these seismic data, to deduce the geometry as well as some of the elastic properties and densities of the materials of the earth through which the seismic energy has propagated and from which it has been reflected.
A seismic reflection from an interface will arrive at a receiver after a two-way travel time, denoted and used herein as, t(X), where X is defined and used herein as the distance between the source and the receiver, or "offset" distance. This "moveout time" t(X) may be used to "dynamically correct" seismic data acquired at an offset distance X so that a reflection is adjusted in time to appear as if it had been acquired at zero offset, i.e., X=0. Conventional "stacking" is accomplished by summing or averaging such dynamically corrected data.
Conventional processing of compressional-wave data uses data collected with many sources and many receivers and gathers the data by the common midpoint (CMP) technique, as illustrated in FIG. 1A. Traces with a common "midpoint" between the source and receiver are collected or gathered at a surface gather point. For example, in FIG. 1A, S.sub.1 and R.sub.1 are the source and receiver pair for the first trace and have a midpoint at the surface point(0). FIG. 1B depicts the corresponding hyperbolic moveout of such data (where the numbers used correspond to the subscripts used in FIG. 1A). That is, after the data have been acquired for a number of sequential source positions the traces for various source-receiver combinations are sorted or gathered into different midpoint groups which have the same or "common" surface location of the "midpoint" between the source and receiver positions (as depicted in FIG. 1A).
This sorted or gathered midpoint data is then analyzed or processed to determine effective velocities V.sub.e, also called normal moveout velocities (or moveout velocities), for reflections from various depths, i.e., various values of t.sub.o. One method is to determine for each a value of V.sub.e which provides dynamic corrections which maximize the resultant amplitudes of the "stacked" data in a time gate around t.sub.o.
The original basis for CMP processing is the fact that each trace in a gather images (or comprises reflections from) approximately the same subsurface points, and, when properly adjusted for differing path times due to the different offsets, the set of corrected traces may be averaged to give an enhanced picture of the reflection response of the earth below that CMP surface location by emphasizing true primary reflections and discriminating against multiple reflections and other undesirable noise.
Thus, seismic data may be processed to obtain panels of seismic sections that extend from the line of acquisition on the surface of the earth down into the earth. For conventional 2 D data, several such "lines" (also referred to herein as "lines of seismic data") may be obtained over an area of interest. These lines of seismic data often intersect and the problem of how to recognize and align common subsurface features at the points of intersection arises. If the common subsurface features are not consistent, i.e., do not align, a mistie results.
Prior attempts to align such subsurface features at points of intersection have consisted mainly of cross-correlation procedures to estimate the time misalignment between intersecting lines of seismic data. However, if the reflection waveforms on two intersecting lines differ in shape, cross-correlation cannot correctly estimate time shifts between such lines. Other attempts generate filters (based on cross-correlations) with large numbers of variables to make one line "look" like the other; this technique can make anything look like anything else, i.e., with large numbers of variables any line can be made to match some other line.
Vertical seismic profile traces (VSP) and well-log synthetic traces are not acquired or processed in the same manner as seismic traces, consequently, using this data in conjunction with lines of seismic data could also result in common subsurface features on the traces not aligning at an "intersection" between the lines of seismic data and the VSP and well-log synthetic traces. Although, the VSP and well-log synthetic traces may not have been acquired directly on the line of seismic data, i.e. intersect with the lines, in this specification, the non-alignment of the this data at an actual intersection or hypothetical intersection, due to disparate processing and acquisition techniques, will also be referred to as a mistie or the data is not consistent.
In addition, although the acquisition and processing of "conventional seismic traces" was heretofore discussed, VSP traces and well-log synthetic traces are not acquired or processed in exactly the same manner. However, since VSP and well-log synthetic traces carry the same representative information and the invention of this specification treats all traces the same, the term "seismic traces" will also encompass both VSP and well-log synthetic traces for this specification,. A "seismic trace" according to this specification is a time varying signal having variations in amplitude and phase representing reflections from subsurface features.